In typical “plasma etching”, fluorocarbons, inert gases, oxygen and the like are used as processing gases, and a high-frequency electric field is applied to such processing gases to cause a glow discharge and generate plasma. Etching is then performed by reacting reactive species in the plasma with a processing subject substrate that includes an etching target. In recent years, various plasma etching methods have been proposed in accompaniment to increasing demand for diversification of semiconductor device applications and refinement of etching patterns. Of these plasma etching methods, a technique referred to as atomic layer etching (hereinafter, also referred to as “ALE”) that enables control of etching shape on the atomic layer level (i.e., on the order of angstroms) is attracting attention.
In plasma etching, a thin layer is deposited on a processing subject substrate and material at the surface of the processing subject substrate or the thin film is etched, it has generally been the case that thin film deposition and etching proceed in a substantially concurrent manner. On the other hand, in the aforementioned ALE method, various controls are performed such that a deposition step of depositing a thin film on a processing subject substrate and an etching step of etching by causing reactive species to collide with the processing subject substrate are switched between and implemented separately. For this reason, it has been possible to control etching shape at the atomic layer level through ALE as previously described.
Note that the thin film deposited in the deposition step may, depending on a component at the deposition location, function as a protective film that protects the processing subject substrate or function as an active film that contributes to etching of the processing subject substrate. For example, the processing subject substrate may include a processing target film that is to be etched to form a pattern and a non-processing target film that is to remain without being etched, and when a thin film is deposited on the non-processing target film, the deposited thin film may function as a protective film in an etching step, whereas when a thin film is deposited on the processing target film, the deposited thin film may function as an active film in an etching step.
However, one problem with ALE is that processing takes longer to complete than with a conventional plasma etching method because of the need to switch between deposition and etching steps. Increasing the etching rate of the etching step has been considered as a means of shortening the processing time, but this may lead to etching of a protective film or a non-processing target film that is not supposed to be etched. Etching of a non-processing target film or protective film makes it difficult to achieve sufficient refinement and high accuracy of an etching pattern.
Consequently, a technique has been proposed (for example, refer to PTL 1) that can inhibit etching of a non-processing target in ALE by using an inert gas that is excited to a metastable state (also referred to as a “metastable gas”). An apparatus described in PTL 1 enables etching of a processing subject substrate using a metastable gas while preventing plasma charged species generated by plasma excitation of an inert gas from reaching the processing subject substrate through a separating plate structure that can prevent movement of plasma charged species.